1. Field of the Invention
The invention relates to direct printing. More particularly, the invention relates to a system and method for non-contact direct printing using conductive particles in an ink solution for dispersion on non-uniform substrates.
2. Background Art
Traditional deposition technologies, such as photolithography and screen-printing, both of which are discussed below, are restricted to planar substrates. FIG. 1 illustrates an emulsion exposure process step for lithography printing according to the prior art, and FIG. 2 illustrates a simplified side view of a lithographic printing system according to the prior art.
The lithography process described in reference to FIGS. 1 and 2, can be used to print on paper 20, or other substrates 60 (see FIG. 7). As shown in FIG. 1, current lithographic processes utilize aluminum plates (plates) 2 that have a brushed, or “roughened” texture. The plates 2 are covered with a smooth photosensitive emulsion 4. A photographic negative 6 of the desired image is laid on top of the plate 2, and is exposed to light 8, transferring a positive image to the emulsion 4. The emulsion 4 is then chemically treated to remove the unexposed portions of the emulsion 4. As shown in FIG. 2, the plate 2 is affixed to a first drum 10 on a printing press, and water 24 is rolled over the plate 2, which adheres to the rough, or negative portions of the image. A ink roller 12 coated with ink 14 is then rolled over the plate 2, which adheres to the smooth, or positive portions of the image. If this image were directly transferred to paper 20, or another type of substrate 60, it would create a positive image, but the paper 20 or substrate 60 would be moistened. Instead, a second drum 18 covered with a rubber surface is rolled over the plate 2, which squeezes away the water 24, and picks up the ink 14. The second drum 18 is then rolled over the paper 20 or substrate 60, transferring the ink 14 to it. Because the image is first transferred to the rubber second drum 18, the process is called “offset lithography,” due to the fact that the image is offset to the second drum 18 before being applied to the paper.
As one of ordinary skill in the art can appreciate, the lithographic or offset printing process described above in reference to FIGS. 1 and 2 is complicated, involves skilled laborers, and requires very costly machinery, which requires expensive set-up and maintenance. In addition to these significant drawbacks, the lithographic or offset printing system does not print well on non-uniform substrates, whether they are paper or other materials. In some circumstances, use of the lithographic or offset printing system could be detrimental to the actual substrate material itself. Thus, there are significant problems with use of the lithographic or offset printing process on many types of substrate materials.
Another type of lithographic “printing” process is the photolithography process, which is the process for transferring geometric shapes on a mask to the surface of a silicon wafer. While this process is well known for making integrated circuits, it can also be used to impart images onto the semiconductor wafers. This process is therefore useful for only one particular type of substrate, and as shall be discussed, also has significant disadvantages. For example, for even applying simple images to a wafer substrate, as might be done on silicon solar cells, there are numerous steps that involve very expensive machines and very well trained personnel to run the expensive machines. The steps involved in the photolithographic process are wafer cleaning; barrier layer formation; photoresist application; soft baking; mask alignment; exposure and development; and hard-baking.
Attention is directed toward FIG. 3. In the first step, the wafers 30 are chemically cleaned to remove particulate matter on the surface as well as any traces of organic, ionic, and metallic impurities. After cleaning, silicon dioxide, which serves as a barrier layer, is deposited on the surface of the wafer. After the formation of the SiO2 layer 28, photoresist 26 is applied to the surface of the wafer 30. High-speed centrifugal whirling of silicon wafers 30 is the standard method for applying photoresist coatings 26 in manufacturing. This technique, known as “spin coating,” produces a thin uniform layer of photoresist 26 on the wafer 30 surface.
There are two types of photoresist (resist) 26: positive and negative. For positive resists, the resist 26 is exposed with UV light 42 (see FIGS. 4A, 4B, and 4C) wherever the underlying material is to be removed. In these resists 26, exposure to the UV light 42 changes the chemical structure of the resist 26 so that it becomes more soluble in the developer. Whatever is exposed, therefore, is more soluble. The exposed resist 26 is then washed away by the developer solution, leaving windows of the bare underlying material. The mask 32, therefore, contains an exact copy of the pattern which is to remain on the wafer 30. See assemblies 34, 36 in FIG. 3.
Negative resists 26 behave in just the opposite manner. Exposure to the UV light 42 causes the negative resist 26 to become polymerized, and more difficult to dissolve. Therefore, the negative resist 26 remains on the surface wherever it is exposed, and the developer solution removes only the unexposed portions. Masks 32 used for negative photoresists, therefore, contain the inverse (or photographic “negative”) of the pattern to be transferred. See assemblies 38, 40 in FIG. 3.
Following the exposure of the photoresist layer 26, the step of soft-baking occurs. Soft-baking is the step during which almost all of the solvents are removed from the photoresist coating 26. Soft-baking plays a very critical role in photo-imaging. The photoresist coatings 26 become photosensitive, or imageable, only after softbaking. Over-soft-baking will degrade the photosensitivity of resists 26 by either reducing the developer solubility or actually destroying a portion of the sensitizer. Under-soft-baking will prevent light from reaching the sensitizer. Positive resists 26 are incompletely exposed if considerable solvent remains in the coating. This under-soft-baked positive resists 26 is then readily attacked by the developer in both exposed and unexposed areas, causing less etching resistance.
One of the most important steps in the photolithography process is mask 32 alignment. A mask or “photomask” 32 is a square glass plate with a patterned emulsion of metal film on one side. The mask 32 is aligned with the wafer 30, so that the pattern can be transferred onto the wafer 30 surface. Each additional mask 32 after the first one must be aligned to the previous pattern.
Once the mask 32 has been accurately aligned with the pattern on the wafer's 30 surface, the photoresist 26 is exposed through the pattern on the mask 32 with a high intensity ultraviolet light 42. There are three primary exposure methods: contact, proximity, and projection. They are shown in FIGS. 4A-4C.
In contact printing, FIG. 4A, the resist-coated silicon wafer 30 is brought into physical contact with the glass photomask 32. The wafer 30 is held on a vacuum chuck, and the whole assembly rises until the wafer 30 and mask 32 contact each other. The photoresist 26 is exposed with UV light 42 while the wafer 30 is in contact position with the mask 32. Because of the contact between the resist 26 and mask 32, very high resolution is possible in contact printing (e.g. 1-micron features in 0.5 microns of positive resist). Some drawbacks do exist, however. For example, debris, trapped between the resist 26 and the mask 32, can damage the mask 32 and cause defects in the pattern.
The proximity exposure method, shown in FIG. 4B, is similar to contact printing except that a small gap, between 10 to 25 microns wide, is maintained between the wafer 30 and the mask 32 during exposure. This gap minimizes (but may not eliminate) mask 32 damage. Approximately 2 to 4 micron resolution is possible with proximity printing.
Projection printing, shown in FIG. 4C, avoids mask 32 damage entirely. An image of the patterns on the mask 32 is projected onto the resist-coated wafer 30, which is many centimeters away. In order to achieve high resolution, only a small portion of the mask 32 is imaged. This small image field is scanned or stepped over the surface of the wafer 30. Projection printers that step the mask image over the wafer surface are called step-and-repeat systems. Step-and-repeat projection printers are capable of approximately one micron resolution. Following exposure, one of the last steps in the photolithographic process is development. FIG. 5 shows response curves for both negative and positive resist after exposure and development.
At low-exposure energies, the negative resist 26 remains completely soluble in the developer solution. As the exposure is increased above a threshold energy Et, more of the resist film 26 remains after development. At exposures two or three times the threshold energy, very little of the resist film 26 is dissolved. For positive resists 26, the resist solubility in its developer is finite even at zero-exposure energy. The solubility gradually increases until, at some threshold, it becomes completely soluble. These curves are affected by all the resist processing variables: initial resist thickness, pre-bake conditions, developer chemistry, developing time, and others. Hard-baking is the final step in the photolithographic process. This step is necessary in order to harden the photoresist and improve adhesion of the photoresist to the wafer surface. See FIGS. 6A and 6B.
As one of ordinary skill in the art can appreciate from the discussion above, there are significant drawbacks for use of photolithography when printing images on semiconductor wafers 30. The discussion above serves to highlight the incredible complexity of the steps involved, the precise and therefore expensive equipment necessary to perform such printing, and the likelihood that small imperfections or problems in manufacturing can drastically reduce throughput.
This photolithographic process is used when performing vacuum deposition onto thick and thin film hybrids. One example of a thin film device is a metal oxide semiconductor field effect transistor (MOSFET), which is used in active matrix liquid crystal displays (AMLCD). Another example of a delicate thin film devices are thin film transistor liquid crystal displays (TFT-LCD's). TFT-LCD's utilize large amounts of transistors. Printing on the transistors can only be accomplished by vacuum deposition. Vacuum deposition makes use of the photolithographic process discussed above. In addition to the expensive photolithographic tools already discussed, vacuum deposition requires additional and expensive machinery, and a significant amount of steps to accomplish the process. Further, skilled workers, which are needed to run the expensive machinery, add to the overall production costs.
FIGS. 7-12 illustrate a process for screen mesh printing according to the prior art. Screen printing is a very old, but commonly used technology that involves relatively inexpensive equipment, but still presents difficulties in practical usage in some applications. As shown in FIG. 7, the screen printing system comprises a screen frame 52 for holding the screen or mesh 66 in place. A stencil 54 is made on the mesh 66 by applying a photosensitive material to the mesh 66, and then applying a negative of the image to be printed onto the photosensitive material. The photosensitive material is developed, leaving a negative of the image to be printed on the mesh 66. Ink 58 is applied to the mesh 66 with the stencil 54, and a squeegee 56 pushes the ink through the parts of the mesh 66 that does not have the stencil 54, onto the substrate 60 below. This process is shown in greater detail in FIG. 8. There are, however, many difficulties that can be encountered when using the screen mesh printing process with non-uniform and other substrate materials.
For example, printing high-resolution patterns into recessed areas presents significant problems with screen-mesh printing. As shown in FIG. 9, the squeegee 56 will have difficulty filling in the valley 68 in the substrate 60 with the ink 58. FIG. 10 illustrates problems associated with screen printing into trenches 68. Again, the squeegee will not be able to adequately fill in, or lay a conductive trace through, the valley/trench 68. In FIG. 11, a mesa 70 exists on the non-uniform substrate 60. The screen printing process cannot adequately print over the mesa 70, thereby leaving portions of the substrate uncovered with the ink 58. Mesas 70 represents the opposite problem as recesses, or valleys 68, but with a further complication. Because the mesa 70 rises above the surface of the substrate 60, it can interfere with operation of the mesh. As shown in FIG. 11, the mesh 66 is pushed up by the mesa 70, causing not only the ink 58 to flow improperly around and on the mesa 70 itself, but perhaps also on the substrate 60 that is in close vicinity to the mesa 70. The closer the mesa 70 is to the squeegee 56, the more sharply pronounced the angle the mesh 66 will make with the surface of the substrate 60, and the greater the chance the ink 58 will not be properly deposited. Other examples of problem areas include printing over sharp edges or ramps (e.g. to connect a contact pad on a die to an electrode on a PCB), or printing a conductive trace between an integrated circuit with lead pins placed on a substrate (PCB) material. In the prior art, the only way to connect the lead pin to the electrode (or pad) on the substrate is with a metal wire.
FIG. 12 illustrates the screen printing process when encountering a via (or “through”) hole 72. Conventional screen printing technology cannot get the ink into the via hole in a substantially consistent manner.
The above discussion highlights the significant difficulties encountered when using screen printing on traditional substrate materials. Advanced printing techniques are simply not possible with screen printing. For example, printing of multi-layered features with conventional mesh and photolithography processes cannot create multi-layered devices, or 3-dimensional (3-d) structures. For example, in mesh printing, the mesh 66 will begin to experience the same problems as when a mesa 66 is encountered when a build up occurs. Or, when the squeegee 56 is applied to the mesh 66, the 3-d object will be destroyed by the very process being used to create it.
Additionally, printing on fragile substrates can be damaged by screen-printing. Very fragile substrates, such as thin Si wafers (solar cells), need additional printing to be performed on them after they have been formed. For solar cells, the manufacturer needs to print the current collector grid on the solar cell (this is where the current created by the silicon solar cell is collected and connected to the power system). Solar cells, being made of silicon, are extremely expensive as they need to be made very large in order to be effective. Because of the significant cost associated with the silicon, however, manufacturers try to make the cells as thin as possible, and hence they are substantially more fragile. Screen printing can damage the fragile solar cells.
Printing on a surface that has a wet, chemically-active coating, such as reducing agent or a fixing agent, presents problems for the screen printing process. Placing a second mesh 66 onto a previously wet surface, whether it has been made wet by a first printing action, or some other chemical process, will negatively affect the first wet surface simply because of the physical interaction of the mesh 66 with the wet surface. Meshes 66 are typically made of metal or some other hard material and will interfere with the wet surface for that reason. Similar to this problem is the problem of printing on substrates that have liquid materials such as previously printed inks. One wet ink 58 on top of another, especially when presented through the use of a squeegee 56, will cause the two to interfere with each other, rendering the printing nearly useless.
Printing on surfaces that have thin film devices such as MOSFET transistors, which are used in active matrix liquid crystal displays (AMLCD), will also present problems with the screen printing process. For example, thin film transistor liquid crystal displays (TFT-LCD's) utilize a large amounts of transistors that are very fragile. The force of the squeegee 56 upon the thin fragile transistor surface can break them.
Thus, a need exists for printing on non-uniform substrates that overcomes all of the above mentioned difficulties, as well as those not mentioned, and provide the advantages described in greater detail below.